The accurate measurement of the frequency and phase of received signals is important in systems such as radar range and range rate measurement, navigation, and collision avoidance systems. It is also useful in communication systems for extracting bit timing information to permit synchronous detection of digital data streams.
Phase locked oscillators (PLO) provide the best techniques for producing accurate measurement of phase and frequency. Basically, a phase locked oscillator compares the phase of a signal-- the received signal in the examples to be discussed below-- to the phase of a voltage controlled oscillator (VCO). The resultant phase difference modifies the control voltage of the VCO to change its frequency in a direction that reduces the phase difference.
The accuracy of a PLO can be increased by using digital techniques. The accuracy of a digital system increases with the number of bits used. Frequency limitations are imposed, however, by the speed of the digital devices employed in the system. A basic feature of the digital PLO systems is the generation of a frequency which is controlled by a digital number. The digital number is a function of the phase difference between the VCO output signal and the received signal.
A common PLO technique is the Programmable-Divide-by-N in which the frequency of a reference oscillator is divided and compared to the divided phase difference between the received signal and the VCO output signal. Varying the division ratios varies the frequency of the VCO. If the VCO frequency (f.sub.s) is divided by N and the reference frequency (f.sub.R) is divided by R, then the VCO output frequency will be EQU f.sub.s = (N/R)f.sub.R.
comparing the phase of the VCO output signal to the phase of the received signal produces a phase difference which is converted to a digital number by an analog-to-digital converter. The resulting digital number is N and controls the frequency of the VCO to track the received signal. Therefore, the value of N will be proportional to the received frequency.
Another technique, called Iterative Synthesizing, combines frequencies obtained by multiple divisions of a plurality of reference frequencies. The combinations are controlled by a digital number so that the resulting output frequency is proportional to the digital number. Practically, iterative synthesizers have high resolution, wide tuning range, and high spectral purity but require a large amount of expensive hardware.
Another technique uses an Incremental Phase Modulator to generate a controlled frequency offset from a given reference frequency. The offset is controlled by a digital number. An example of the implementation of this technique would be a tapped delay line, the output signal from each tap being coupled to a separate gate each of which is enabled by a different one of the bits in the controlling digital number. The output signals from all the gates would be combined in a mixer to produce the output frequency. This implementation is called a Serradyne. The frequency at which this type of system operates is determined by the number of taps, phase quantization, and the stability of the tapped delay line. The complexity of the implementation makes this technique unattractive for high resolution systems.
Another approach uses arithmetic techniques to generate the controlling voltage for the VCO. One such system is described in application Ser. No. 462,772 by Bosselaers and assigned to the same assignee as this application. In this technique, an arithmetic unit adds and subtracts programmed counter values as determined by a reference a-c signal and a variable a-c signal derived from the VCO output signal. The output signal from the arithmetic unit is converted from a digital to an analog signal and filtered to apply a controlling voltage to the VCO.
The prior art techniques described above have certain disadvantages, the importance of which depends on the use of a given technique. Specifically, the disadvantages include variations in frequency of the VCO caused by temperature, component value drift, and so on. The VCO drift is compensated for by the loop action and is not apparent, but the produced frequency jacks repeatibility. That is, if a digital register value is proportional to the frequency at one point in time, it will not necessarily have the same proportionality at another point in time because of VCO frequency drift.
The frequency being measured is not represented directly by any digital number in the system (excepting the iterative synthesizer). The frequency of the VCO is controlled by a frequency determining device such as crystal, a tank circuit, or the like. The digital numbers in such systems represent the departure from the center (base or nominal) frequency but, because of oscillator drift, neither the exact frequency nor exact frequency change is known. The techniques described above require extensive hardware with resulting system complexity. This not only increases manufacturing cost but adds to the maintenance cost. The techniques described above do not exhibit phase coherency. As the frequency changes, the loops unlock; some even oscillate (hunt) around the new frequency.
The invention disclosed and described herein is a system that does not require a VCO, avoiding the frequency anomolies associated therewith. Direct digital readout of the frequency or its change (doppler) is available. The system of the invention is simple to construct and exhibits phase coherency. Its accuracy is limited only by external factors such as the system clock.